1. Field of the Invention
The present invention relates to a video clock generating circuit used in an image forming apparatus using a laser beam, such as a laser beam printer and a digital copying machine, for generating a video clock for defining a data processing operation for each pixel. Furthermore, the present invention relates to a horizontal synchronizing signal generating device for generating a horizontal synchronizing signal for synchronizing laser beam scanning and image data processing.
2. Description of the Prior Art
In a laser beam printer conventionally used, an image is printed as follows. A photoreceptor is scanned by a laser beam subjected to modulation based on data corresponding to an image to be printed. Then, an electrostatic latent image formed on the photoreceptor is developed into a toner image, and the toner image is transfered and fixed to paper. In such a laser beam printer, the scanning of the photoreceptor by the laser beam and the modulation of the laser beam based on the image data must be synchronized with each other. Therefore, a beam detection signal corresponding to the timing of the scanning of the photoreceptor by the laser beam is generated by, for example, detecting the laser beam in a predetermined position, and a so-called horizontal synchronizing signal which is synchronized with the beam detection signal is generated. This horizontal synchronizing signal is synchronized with the laser beam scanning. Accordingly, if the laser beam is subjected to the modulation corresponding to the image data in accordance with a video clock inside of the laser beam printer on the basis of the horizontal synchronizing signal, an electrostatic latent image corresponding to a desired image can be formed on the photoreceptor. The above described video clock is a clock for defining a processing operation corresponding to image data for each pixel, which is generally generated independently of the beam detection signal.
FIG. 11 is a timing chart for explaining the technique of generating a horizontal synchronizing signal which has been conventionally used in the laser beam printer. Line (a) of FIG. 11 shows a beam detection signal, FIG. 11 (b) shows a video clock, and line (c) of FIG. 11 shows a horizontal synchronizing signal. In this conventional technique, the timing (time t1) of the first rising of the video clock after the falling of the beam detection signal is detected. The horizontal synchronizing signal is so generated as to fall in synchronization with the first rising of the video clock after the falling of the beam detection signal and to have a pulse width corresponding to a constant number of periods of the video clock from this falling.
In the above described conventional technique, a time interval .DELTA.t between the falling of the beam detection signal and the falling of the horizontal synchronizing signal depends on the relationship between the beam detection signal and the video clock. Therefore, variation corresponding to a maximum of one period of the video clock may occur in the time interval .DELTA.t. This variation causes the time relationship between the laser beam scanning and the horizontal synchronizing signal to differ for each scanning line formed on the photoreceptor by the laser beam. Consequently, the image formed on the paper is shifted by a maximum of one pixel between a plurality of scanning lines.
In order to reduce the shift of the image, the frequency of the video clock for synchronizing the beam detection signal and the horizontal synchronizing signal must be significantly increased. In this case, however, the processing speed of a horizontal synchronizing signal generating circuit is significantly increased, thereby to make it difficult to design the circuit, and a high frequency signal is handled, thereby to make it very difficult to take measurements against noise.